Abstract

Alien species invasion of the forest ecosystem may lead to structuring of the plant community structure. Invasive Acacia is one of the alien tree species in forests. The ecological relationship between Acacia melanoxylon invasion and forest ecology is not well understood in tropical humid forest ecosystems. Therefore, the main objective of this study was to determine the influence of A. melanoxylon on two commercial forest tree species (Cupressus lusitanica and Pinus patula) in a humid tropical forest (North Tinderet Forest, in Kenya). Transects measuring 500 m long were used for sampling the uninvaded and invaded sites. Along the transect, three 10 m × 10 m plots were systematically placed at 235 m intervals to enable counting and recording of trees (density), measuring diameter at breast height (DBH) > 1.3 m) as well as tree heights. The study established that the tree density, DBH and height of Pinus patula and Cupressus lusitanica, were significantly (P < 0.05) higher at the non-invaded sites compared to the invaded sites. These findings suggest that A. melanoxylon invasion reduced the growths and establishment of commercial tree species. It is thus recommended that future studies on ecological conditions for growth of A. melanoxylon should be conducted in controlled environment through growth response measurements which was not possible under the current study. Invasion by A. melanoxylon reduce the growth of commercial plantation tree species (Pinus patula and Cupressus lusitanica) therefore strategies for controlling growth of the A. melanoxylon should be done. Future studies should investigate how the nutrient uptake by the commercial plantation tree species is influenced by Acacia melanoxylon density.

1. Introduction

There are a number of invasive plant species which have been translocated into new environments, where they have survived, propagated and eventually dominated the alien landscapes ^{1, 2}. In some instances, there are accidental introductions of species that tend to be invasive ³. Invasive species belonging to the genus Acacia are widespread in the global biomes except in the Arctic and Antarctic regions ^{4, 5, 6}. The number of acacia species appearing outside their natural range and currently considered invasive are about 22-25 species ^{7, 8}.The genus occurs across a wide range of ecosystems, from arid and semi deserts, afro-montane forests, savannah grassland and in plantation forests ⁹. In the case of Acacia melanoxylon, it occurs, and can grow and survive in almost all the continents in countries such as Europe ¹⁰, Oceania ^{10, 11, 12}, Africa ^{13, 14}, South America ¹⁵, and Asia ¹⁶. In their new locations, A. melanoxylon establish rapidly, grow and dominate the surrounding environments ^{17, 18}.

A key aspect of the invasive biology of the A. melanoxylon is the knowledge of their impacts on the native tree species. Such impacts can be multidirectional as woody alien species can increase or decrease the ecological features of the native tree species ^{19, 20, 21}. The proliferation of A. melanoxylon as an invasive species may alter the trophic structures, and change the plant density, composition, biomass and growth of trees ^{22, 23} The structuring effects of invasive species result into partial displacement or complete replacement of the native plant species with the invader ^{19, 24}. In a forest, the A. melanoxylon has history of affecting growth and proliferation of the trees in adjoining forest stand ^{23, 24} Limited studies of A. melanoxylon on forest growth ^{25, 26}, reveal four mechanisms of invasion such as prolific seeding, fast reproduction; high survival ability, and speedy regeneration ^{27, 28}. These mechanisms allow for the invasive A. melanoxylon to overwhelm other vegetation establishment and outcompete other forest vegetation in nutrient ²⁹. Information on the influence of these acacia species may be influenced by the type of species invading, habitat conditions, types of trees invaded as well as the environmental conditions ^{30, 31, 32, 33}

There is an increasing proliferation of invasive species in the plantation forests in Kenya ³⁴, which may create a challenge associated with such invasion. There is a tendency of the invasive A. melanoxylon to affect tree growth, native vegetation establishment and soil quality. Despite this, there are no meaningful studies conducted in Kenya’s plantation forest ecosystems to determine the response of trees, vegetation and soils to A. melanoxylon invasion. Therefore, this study aims to determine the influence of A. melanoxylon on two commercial forest tree species (Cupressus lusitanica and Pinus patula) in the North Tinderet Forest, North Rift in Kenya. We hypothesize that commercial timber tree species, C. lusitanica and P. patula within the proximity of invasive species such as A. melanoxylon will manifest in reduced growth or decline productive potential assuming all other factors remain constant for a given site.

2. Materials and Methods

The study was conducted in the Nabkoi and Timboroa forests situated within the North Tinderet Forest in Uasin Gishu County (Kenya) (Figure 1). It covers a total area of 3,345.2 km² with a population of 1,163,000 according to the 2019 Kenya National Housing and Demographic Survey Census ³⁵. The areas occur within longitude 34º50’E to 35º37’E and latitudes 0º03’S to 0º 55’N.

The climate of the area is typically tropical rainforest. The altitude range between 2600 to 2950 m asl however, Timboroa Forest ((2913 m asl) is at higher altitude compared to Nabkoi Forest ((2634 m asl). Annual rainfall range between 1,328 to 1,405 mm. The cyclic pattern of rainfall is long rainy season between March to June, dry season between July and September, short rainy season from October to November, followed by another dry season from December to February. Temperatures range from a minimum of 7ºC (June–August) to a maximum of 29ºC. The two study sites have exotic plantation species, mostly dominated by Cupressus lusitanica (Timboroa) and Pinus patula (Nabkoi). Within the mosaic of indigenous vegetation and the above commercial timber tree species is Acacia melanoxylon which appears to be spreading in the area including within cypress and pine plantations.

Geology is mainly composed of basalt rock boulders of pre-Cambrian formations. There are two main soil types: Soils of the plateaus (Ferralsols) - deep red well-drained soils – red and brown loam as well as the soils of the bottomlands (gleysols) - poorly drained soils - red and brown clay. The topsoil layer is mainly red loam soils rich in organic matter hence susceptible to erosion but suitable for agricultural activities and tree growing.

Field sites were selected based on altitude, slope, and aspect. The study area was divided into two specific study sites from which data was collected. Nabkoi (2634 m asl) and Timboroa (2913 m asl) were chosen to represent the species of interest because of their difference in altitude and proximity (3-4 km apart) within the same forest formation of the study area. For each of the two study sites (Nabkoi and Timboroa) (i) distinct stands Acacia melanoxylon, largely taken over natural vegetation, and (ii) stands for each of the two species of interest, Cupressus lusitanica and Pinus patula were appropriately identified and selected. The uninvaded and invaded stands of each species were compared, at the two sites. In summary, Nabkoi (Acacia melanoxylon stands, Cupressus lusitanica invaded and uninvaded stands; Pinus patula invaded and uninvaded stands and the same process repeated in Timboroa).

The fieldwork was carried out from January 2022 to February 2023 covering both dry and rainy seasons. A reconnaissance survey was done in November to December 2021 preceding the fieldwork to identify forest sites to be assessed. The method of systematic sampling was used to select sampling units. In the selected stands, three line transects were established taking into account slope orientation to capture horizontal and vertical variation. On each transect, a 10 m × 10 m plots were established located at 235 m intervals from which measurements were taken (Figure 2). From each of the nine sample plots, assessments of trees were done.

Every tree in the sample plot (10 × 10 m) had its Diameter at breast height (DBH) measured, i.e., at 1.3 meters above the ground using a diameter tape, taking the necessary precautions not to slant, or loosen the tape and measurements recorded in specially designed data collection forms labelled to indicate site, species (invaded or uninvaded) and plot number. Tree heights were also measured using a Suunto clinometer (by selecting one of the scales on the instruments at 1:15 or 1:20) and measurements appropriately recorded

Data collected subjected to statistical analysis using STATISTICA 8.0 ³⁶ and SPSS 23.0 Statistical Packages ³⁷. In addition, descriptive statistics was used, mean tree dbh, height, diameter size distribution and, plot densities (on a per hectare basis) were used to depict general trends. Densities of shrubs and lianas, and herbaceous plants at the plot level were expressed as the number of individual plants per square metre. The spatial variation in plant density, DBH and height were analyzed using One-way ANOVA and post discrimination of significantly different means done using Duncan’s Multiple Range Test (DMRT).

3. Results

Stand densities for C. lusitanica and P. patula invaded and those not-invaded (uninvaded) by A. melanoxylon are represented in Figure 3). Plantation density for these two species at planting is usually 1600 stems/ha. For both species invaded stands had lower densities than uninvanded ones regardless of the site. Whereas Pinus patula performed better in the higher altitude of Timboroa (showing relatively higher levels), the converse was true for Cupressuss lusitanica. Stands of C. lusitanica had comparatively higher densities in Nabkoi than in Timboroa.

The average tree diameter at the plot level was clearly higher for uninvaded stands than for those invaded by A. melanoxylon (Figure 4). Average plot diameters for uninvaded stands of C. lusitanica at the uninvaded were 20-30 cm while at the invaded average tree diameters at the invaded stands were 10-12 cm in Nabkoi. In Timboroa, the species plot diameters for the uninvaded stand stood at 30 to 40cm, while plot diameters at the invaded stand were comparable ranging from 25 to 35cm (suppression growth level much less than in Nabkoi). The overall, C. lusitanica plot diameters were significantly higher at the uninvaded stands than in invaded ones for both Nabkoi and Timboroa forest sites (F_{(1, 76)} = 14.411, df = 1, P = 0.0002).

Pinus patula stands followed more or less the same pattern (Figure 5). In Nabkoi uninvaded stand plot diameters were from 20 to 30 cm, while in the invaded stand, they were at a lower level while corresponding to 10-12 cm invaded site. In Timboroa, the uninvaded stand plot diameters ranged from 35 to 45 cm while in invaded were again at a lower level of 25 to 35cm. Similar to C. lusitanica, plot diameters for P. patula, in uninvaded stands for Nabkoi and Timboroa are significantly higher than invaded ones (F_{(2, 76)} = 15.412, P = 0.0002).

Tree diameter size-frequency distributions for two species and study sites are depicted in Figure 6. Most C. lusitanica trees in the Nabkoi uninvaded stand were recorded in diameter class 21-30 cm while trees in the invaded stand were considerably much smaller within the diameter class of 11-20 cm. In Timboroa on the other hand, the majority of C. lusitanica trees in the uninvaded stand were found within diameter class 31 to 40 cm, and in invaded 21 to 30 cm. It was evident for C. lusitanica Timboroa had larger trees than in Nabkoi for in the uninvaded and invaded stands correspondingly.

The diameter class distribution of P. patula at the uninvaded and invaded sites is shown in Figure 7. The distribution of most P. patula at the Nabkoi uninvaded and invaded site was 11-20 cm. On the other hand, the majority of the P. patula at Timboroa uninvaded forest site aggregated towards 31 to 40 cm while at the invaded sites the aggregation was towards 21 to 30 cm.

The relationship between diameter size distribution and the density of Acacia melanoxylon is shown in Figure 8. There was a significant negative correlation between the diameter sizes of the timber species in relation to Acacia melanoxylon density at all the forest sampling sites. As the density of A. melanoxylon increased, the average tree size of the plantation species decreased.

The tree plot height of C. lusitanica of uninvaded and invaded by A. melanoxylon is provided in Figure 9. The average height of C. lusitanica at the uninvaded sites in Nabkoi aggregated towards 28 to 35 m in uninvaded sites while at the invaded site it was 18 to 24 m. In Timboroa, the uninvaded sites had tree heights aggregated at 20 to 25 m while at the invaded sites most of the tree species aggregated at 18 to 21 cm. The height of C. lusitanica, at the uninvaded sites, was significantly higher than at invaded stands (F_{(2, 76)} = 4.143, P = 0.0012).

The tree plot height of P. patula at the four forest stands (uninvaded and invaded) is provided in Figure 10. The average height of P. patula at the uninvaded sites in Nabkoi aggregated towards 12 to 18 m, while at the invaded site it was 15 to 22 m. In Timboroa, uninvaded and invaded sites had tree heights within 20 to 25 m. The height of P. patula, at the invaded sites, was significantly higher (F = 4.112, P = 0.0042 in Nabkoi but no differences in height occurred in Timboroa. Invasion could trigger competition for light increasing height growth.

The relationship between the height size distribution of commercial tree species and the density of A. melanoxylon at invaded sites is shown in Figure 11. There was a significant negative correlation between the heights of commercial tree species with A. melanoxylon density at all the forest sampling sites.

4. Discussion

The tree density of both Cupressus lusitanica and Pinus patula was higher at the uninvaded sites compared to the invaded sites in the Nabkoi and Timboroa forest sites. This suggests that invasion by A. melanoxylon depressed the tree density by causing higher mortality in invaded stands all other factors being constant. One way in which invasive wood tree species affect resident tree species' biomass is through decimation or extermination. Tree species decimation due to invasion results in the killing of large local tree species by invasive tree species. In comparison with other invasive species, plantations of A. saligna have demonstrated a higher capacity to decimate many other native tree species through allelopathy ³⁸. There are also reports of decimated native tree species by invasive Acacia caven in Argentina ³⁹. Acacia tortilis has also a long record of eliminating the native biomass of plant species through smothering effects ⁴⁰.

The diameter size distribution of C. lusitanica and Pinus patula at the uninvaded sites (Nabkoi and Timboroa) was higher than at the invaded sites. Furthermore, there was a negative correlation between the diameter sizes of all tree species with Acacia melanoxylon density at all the forest sampling sites. These observations suggest that the invasion of A. melanoxylon reduced diameter growth through increased soil and water resources. Given the planting density of 1600 trees /hectare, and stands subjected same silvicultural treatments, new trees occupying the site (by invasion) would lead to increased competition as compared to where no invasion exists. Numerous studies have documented the negative impacts of invasive woody species on tree species ^{8, 41, 42}. In general, the influence of acacia on other tree species' growth has reported to be antithetical to optimal growth of other species. Invasion by A. dealbata has on several occasions reduced plant growth ^{43, 44}. Acacia saligna in the Nile Delta Coast of Egypt was reported to cause negative growth of crops and trees in areas of occurrence ⁴⁵. The invasive Acacia mangium negatively affected the growth of two plant species in paddies ⁴⁶. There are also records of invasive A. auriculiformis threatening growth in DBH of mangrove trees ⁴⁷. Acacia melanoxylon has also been established to negatively affect the growth of Lactuca sativa ⁴⁸, which concurs with the current finding.

Generally, the height of both Cupressus lusitanica and Pinus patula was higher at the uninvaded sites compared to the invaded sites in Nabkoi and Timboroa. Also, there was a negative relationship between the heights of commercial tree species with Acacia melanoxylon density at all the forest sampling sites. This suggests that invasion by A. melanoxylon inhibited the growth of the two plantation species in forests of Uasin Gishu. Studies have documented the negative impacts of invasive woody species on the height of tree species in several environments ^{8, 41, 42}. The suppression of growth in the height of other trees by A. dealbata has been reported ^{43, 44} Acacia saligna also reduced the growth in height of several tree species in the area of occurrence ⁴⁵. There are also records of invasive A. auriculiformis threatening the height of mangrove trees ⁴⁷. Acacia melanoxylon has also been established to negatively affect the growth in height of Lactuca sativa ⁴⁸, which concurs with the current funding. Therefore, it is probable that the observed reduction in height of commercial plantation tree species responded to the occurrence of invasive A. melonaxylon.

In conclusion, the density of Pinus patula and Cupressus lusitanica was higher at the non-invaded site than non invaded sites. It was also observed that the invasion of the forest blocks negatively affected the tree growth and establishments of Pinus patula and Cupressus lusitanica. The density, DBH, the average height and volume of Pinus patula and Cupressus lusitanica tree species, were higher at the non-invaded sites compared to the invaded sites. While A. melanoxylon may be a suitable candidate to provide suites of environmental benefits to the local communities, there is need to control its proliferation in the commercial forest stands.

Competing Interests

The authors declare that they have not competing interest in publication of this manuscript.

References